This invention relates to tangentially fired, fossil fuel furnaces, and more specifically, to firing systems for reducing the NO.sub.x emissions from tangentially fired, pulverized coal furnaces.
Pulverized coal has been successfully burned in suspension in furnaces by tangential firing methods for a long time. The technique known as tangential firing involves introducing the fuel and air into a furnace from the four corners thereof so that the fuel and air are directed tangent to an imaginary circle in the center of the furnace. This type of firing has many advantages, among them being good mixing of the fuel and the air, stable flame conditions, and long residence time of the combustion gases in the furnaces.
Recently though, more and more emphasis has been placed on the minimization as much as possible of air pollution. To this end, most observers in the United States expect the U.S. Congress to enact comprehensive air emission reduction legislation by no later than the end of 1990. The major significance that such legislation will have is that it will be the first to mandate the retrofitting of NO.sub.x and SO.sub.x controls on existing fossil fuel fired units. Heretofore, prior laws have only dealt with the new construction of units.
With further reference in particular to the matter of NO.sub.x control, it is known that oxides of nitrogen are created during fossil fuel combustion by two separate mechanisms which have been identified to be thermal NO.sub.x and fuel NO.sub.x. Thermal NO.sub.x results from the thermal fixation of molecular nitrogen and oxygen in the combustion air. The rate of formation of thermal NO.sub.x is extremely sensitive to local flame temperature and somewhat less so to local concentration of oxygen. Virtually all thermal NO.sub.x is formed at the region of the flame which is at the highest temperature. The thermal NO.sub.x concentration is subsequently "frozen" at the level prevailing in the high temperature region by the thermal quenching of the combustion gases. The flue gas thermal NO.sub.x concentrations are, therefore, between the equilibrium level characteristic of the peak flame temperature and the equilibrium level at the flue gas temperature.
On the other hand, fuel NO.sub.x derives from the oxidation of organically bound nitrogen in certain fossil fuels such as coal and heavy oil. The formation rate of fuel NO.sub.x is strongly affected by the rate of mixing of the fuel and air stream in general, and by the local oxygen concentration in particular. However, the flue gas NO.sub.x concentration due to fuel nitrogen is typically only a fraction, e.g., 20 to 60 percent, of the level which would result from complete oxidation of all nitrogen in the fuel. From the preceding it should thus now be readily apparent that overall NO.sub.x formation is a function both of local oxygen levels and of peak flame temperatures.
Continuing, some changes have been proposed to be made in the standard technique of tangential firing. These changes have been proposed primarily in the interest of achieving an even better reduction of emissions through the use thereof. One such change resulted in the arrangement that was the subject matter of U.S. Pat. application, Ser. No. 786,437, now abandoned, entitled "A Control System And Method For Operating A Tangentially Fired Pulverized Coal Furnace", which was filed on Oct. 11, 1985 and which was assigned to the same assignee as the present Pat. application. In accordance with the teachings of the aforesaid U.S. patent application, it was proposed to introduce pulverized coal and air tangentially into the furnace from a number of lower burner levels in one direction, and to introduce coal and air tangentially into the furnace from a number of upper burner levels in the opposite direction As a consequence of utilizing this type of arrangement, it was alleged that better mixing of the fuel and air was accomplished, thus permitting the use of less excess air than with a normal tangentially fired furnace, which, as is well-known to those skilled in this art, is generally fired with 20-30% excess air. The reduction in excess air helps minimize the formation of NO.sub.x which as noted previously herein is a major source of air pollution from coal-fired furnaces. The reduction in excess air also results in increased efficiency of the Furnace. Although the firing technique to which the aforesaid U.S. patent application was directed reduces NO.sub.x there were some disadvantages associated therewith. Namely, since the reverse rotation of the gases in the furnace cancel each other out, the gases flow in a more or less straight line through the upper portion of the furnace, thereby increasing the possibility of unburned carbon particles leaving the furnace due to reduced upper furnace turbulence and mixing. In addition, slag and unburned carbon deposits on the furnace walls can occur. These wall deposits reduce the efficiency of heat transfer to the water-cooled tubes lining the walls, increases the need for soot slowing, and reduces the life span of the tubes.
Another such change resulted in the arrangement that forms the subject matter of U.S. Pat. No. 4,715,301 entitled "Low Excess Air Tangential Firing System", which issued on Dec. 29, 1987 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 4,715,301, a furnace is provided in which pulverized coal is burned in suspension with good mixing of the coal and air, as in the case of the now abandoned U.S. patent application that has been the subject of discussion hereinabove. Furthermore, all of the advantages previously associated with tangentially fired furnaces are obtained, by having a swirling, rotating fireball in the Furnace. The walls are protected by a blanket of air, reducing slagging thereof. This is accomplished by introducing coal and primary air into the furnace tangentially at a first level, introducing auxiliary air in an amount at least twice that of the primary air into the furnace tangentially at a second level directly above the first level, but in a direction opposite to that of the primary air, with there being a plurality of such first and second levels, one above the other. As a result of the greater mass and velocity of the auxiliary air, the ultimate swirl within the furnace will be in the direction of the auxiliary air introduction. Because of this, the fuel, which is introduced in a direction counter to the swirl of the furnace, is forced after entering the unit to change direction to that of the overall furnace gases. Tremendous turbulent mixing between the fuel and air is thus created in this process. This increased mixing reduces the need for high levels of excess air within the furnace. This increased mixing also results in enhanced carbon conversion which improves the furnace's overall heat release rate while at the same time reducing upper furnace slagging and fouling. The auxiliary air is directed at a circle of larger diameter than that of the fuel, thus forming a layer of air adjacent the walls. In addition, overfire air, consisting essentially of all of the excess air supplied to the furnace, is introduced into the furnace at a level considerably above all of the primary and auxiliary air introduction levels, with the overfire air being directed tangentially to an imaginary circle, and in a direction opposite to that of the auxiliary air.
Yet another such change resulted in the arrangement for firing pulverized coal as a fuel with low NO.sub.x emissions that forms the subject matter of U.S. Pat. No. 4,669,398, entitled "Pulverized Fuel Firing Apparatus", and which issued on June 2, 1987. In accordance with the teachings of U.S. Pat. No. 4,669,398, an apparatus is provided which is characterized by a first pulverized fuel injection compartment in which the combined amount of primary air and secondary air to be consumed is less than the theoretical amount of air required for the combustion of the pulverized fuel to be fed as mixed with the primary air to a furnace, by a second pulverized fuel injection compartment in which the combined primary and secondary air amount is substantially equal to, or, preferably, somewhat less than, the theoretical air for the fuel to be fed as mixed with the primary air, and by a supplementary air compartment for injecting supplementary air into the furnace, the three compartments being arranged close to one another. The gaseous mixtures of primary air and pulverized fuel injected by the first and second pulverized fuel injection compartments of the apparatus are mixed in such proportions as to reduce the NO.sub.x production. Moreover, the primary air-pulverized fuel mixture from the second pulverized fuel injection compartment, which alone can hardly be ignited stably, is allowed to coexist with the flame of the readily ignitable mixture from the first pulverized fuel injection compartment to ensure adequate ignition and combustion. An apparatus is thus allegedly provided for firing pulverized fuel with stable ignition and low NO.sub.x production.
Secondly, the apparatus in accordance with the teachings of U.S. Pat. No. 4,669,398 is characterized in that additional compartments for issuing an inert fluid are disposed, one for each, in spaces provided between the three compartments. The gaseous mixtures of primary air and pulverized fuel are thus kept from interfering with each other by a curtain of the inert fluid from one of the inert fluid injection compartments, and the production of NO.sub.x from the gaseous mixtures that are discharged from the first and second pulverized fuel injection compartments allegedly can be minimized. Also, the primary air-pulverized fuel mixture from the first pulverized fuel injection compartment and the supplementary air from the supplementary air compartment are prevented from interfering with each other by another curtain of the inert fluid from another compartment. This allegedly permits the primary air-pulverized fuel mixture to burn without any change in the mixing ratio, thus avoiding any increase in the NO.sub.x production.
Yet still another change resulted in the arrangement for firing pulverized coal as a fuel while at the same time effecting a reduction in NO.sub.x and SO.sub.x emission that forms the subject matter of U. U.S. Pat. No. 4,426,939, entitled "Method Of Reducing NO.sub.x and SO Emission", which issued on Jan. 24, 1984 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 4,426,939, a furnace is fired with pulverized coal in a manner that reduces the peak temperature in the furnace while still maintaining good flame stability and complete combustion of the fuel. The manner in which this is accomplished is as follows. Pulverized coal is conveyed in an air stream towards the furnace. In the course of being so conveyed, the stream is separated into two portions, with one portion being a fuel rich portion and the other portion being a fuel lean portion. The fuel rich portion is introduced into the furnace in a first zone. Air is also introduced into the first zone in a quantity insufficient to support complete combustion of all of the fuel in the fuel rich portion. The fuel lean portion, on the other hand, is introduced into the furnace in a second zone. Also, air is introduced into the second zone in a quantity such that there is excess air over that required for combustion of all of the fuel within the furnace. Lastly, lime is introduced into the furnace simultaneously with the fuel so as to minimize the peak temperature within the furnace thereby to also minimize the formation of NO.sub.x and SO.sub.x in the combustion gases.
Although firing systems constructed in accordance with the teachings of the now abandoned U.S. patent application and the three issued U.S. patents to which reference has been made heretofore have been demonstrated to be operative for the purpose for which they have been designed, there has nevertheless been evidenced in the prior art a need for such firing systems to be further improved if through the use thereof NO.sub.x emissions are to be reduced to the levels which would be required to be met under the proposed new legislation being contemplated by the U.S. Congress. A need is thus being evidenced in the prior art for a new and improved firing system that would be applicable, in particular, for use in tangentially fired, pulverized coal furnaces to achieve NO.sub.x emission reductions of as much as 50% to 60% from that which would otherwise be emitted from such furnaces which are equipped with prior art forms of firing systems. Moreover, there has been evidenced in the prior art a need for such a new and improved firing system that would be particularly characterized in a number of respects. To this end, one such characteristic which such a new and improved firing system would desirably possess is the capability of establishing through the use thereof several layers of fuel-rich zones in the furnace burner area. Such an arrangement facilitates immediate ignition and associated high temperature with the concomitant effect that release of the organically-bound nitrogen from the coal is introduced into the large fuel-rich zones. Another characteristic which such a new and improved firing system would desirably possess is the ability to achieve through the use thereof both stabilization of the fuel front and the initial devolatilization within the fuel-rich zones of the fuel-bound nitrogen whereby the fuel-bound nitrogen is converted in the fuel-rich zones to N.sub.2. A third characteristic which such a new and improved firing system would desirably possess is the capability of providing through the use thereof "boundary air" to protect the furnace walls from the reducing atmospheres that are known to exist within the furnace when the furnace is in operation. A fourth characteristic which such a new and improved firing system would desirably possess is the capability of providing through the use thereof sufficient overfire air to permit the completion of efficient combustion of the fuel rich furnace gases before these gases reach the convective pass of the furnace. The objective, which is sought to be realized in this regard, is that of ensuring both that the coal combustion process is completed and that the amount of unburned carbon is minimized.
To thus summarize, a need has been evidenced in the prior art for such a new and improved firing system that would be particularly suited for use in connection with tangentially fired, fossil fuel furnaces and that when so employed therein would render it possible to accomplish through the use thereof reductions in the level of NO.sub.x emissions to levels that are at least equivalent to if not better than that which is currently being contemplated as the standard for the U.S. in the legislation which is being proposed. Moreover, such results would be achievable with such a new and improved firing system without the necessity of requiring for the operation thereof any additions, catalysts or added premium fuel costs. In addition, such results would be achievable with such a new and improved firing system that incorporates provisions for eliminating waterwall corrosion which is commonly associated with the reducing atmosphere that is produced during deep staged combustion operation. Furthermore, such results would be attainable with such a new and improved firing system which is totally compatible with other emission reduction-type systems such as limestone injection systems, reburn systems and selective catalytic reduction (SCR) systems that one might seek to employ in order to accomplish even additional emission reduction. Last but not least, such results would be attainable with such a new and improved firing system which is equally suitable for use either in new applications or in retrofit applications.
It is, therefore, an object of the present invention to provide a new and improve NO.sub.x emission reducing firing system for use in fossil fuel-fired furnaces.
It is a further object of the present invention to provide such a NO.sub.x emission reducing firing system for furnaces that is particularly suited for use in tangentially-fired, pulverized coal furnaces.
It is another object of the present invention to provide such a NO.sub.x emission reducing firing system for furnaces which is characterized in that through the use thereof NO.sub.x emissions are capable of being reduced to levels that are at least equivalent to if not better than that which is currently being contemplated as the standard for the U.S. in the legislation being proposed.
It is still another object of the present invention to provide such a NO.sub.x emission reducing firing system for furnaces which is characterized in that through the use thereof NO.sub.x emission reductions are capable of being achieved of as much as 50% to 60% from that which would otherwise be emitted from furnaces which are equipped with prior art forms of firing systems.
Another object of the present invention is to provide such a NO.sub.x emission reducing firing system for furnaces which is characterized in that through the use thereof several layers of fuel-rich zones are established in the furnace burner area.
A still another object of the present invention is to provide such a NO.sub.x emission reducing firing system for furnaces which is characterized in that through the use thereof immediate ignition and associated high temperature are facilitated with the concomitant effect that release of the originally-bound nitrogen from the pulverized coal being fired in the furnace is introduced into the large fuel-rich zones.
A further object of the present invention is to provide such a NO.sub.x emission reducing firing system for furnaces which is characterized in that through the use thereof there is accomplished stabilization of the flame front as well as the initial devolatilization within the fuel-rich zones of the fuel-bound nitrogen whereby the fuel-bound nitrogen is converted to N.sub.2 in the fuel-rich zones.
A still further object of the present invention is to provide such a NO.sub.x emission reducing firing system for furnaces which is characterized in that through the use thereof sufficient overfire air is provided to permit the completion of efficient combustion of the fuel rich furnace gases before these gases reach the convective pass of the furnace.
Yet an object of the present invention is to provide such a NO.sub.x emission reducing firing system for furnaces which is characterized in that through the use thereof no additions, catalysts or added premium fuel costs are needed for the operation thereof.
Yet a further object of the present invention is to provide such a NO.sub.x emission reducing firing system for furnaces which is characterized in that provisions are incorporated therein for eliminating waterwall corrosion which is produced during deep staged combustion operation.
Yet another object of the present invention is to provide such a NO.sub.x emission reducing firing system for furnaces which is characterized in that it is totally compatible with other emission reducing-type systems such as limestone injection systems, reburn systems and selective catalytic reduction (SCR) systems that one might seek to employ in order to accomplish additional emission reduction.
Yet still another object of the present invention is to provide such a NO.sub.x emission reducing firing system for furnaces which is characterized in that it is equally well suited for use either in new applications or in retrofit applications.